Nanofiltration (NF) is classified in the category between ultrafiltration (UF) and reverse osmosis (RO), and refers generally to a filtration technology involving use of a pressure driven membrane with pore size in the range of about 0.5 nm to 5 nm in diameter.
In the water treatment area, NF is generally used if high salt rejection of RO is not necessary. NF membranes may be used in diverse fields, such as water industry for water softening, color removal, heavy metal recovery, food industry, as well as pharmaceutical and biomedical industries. This has resulted in a rapid increase in global market for NF membranes from $89 million in 2006 to an estimated value of $310 million in 2012.
In general, a NF membrane consists of a selective skin layer supported by a porous substrate which provides necessary mechanical strength. The selective skin layer may be fabricated by integrally connecting it to the support layer. Such examples include integral asymmetric polybenzimidazole (PBI) and cellulose acetate NF hollow fiber membranes. This method requires a delicate polymer dope formula and a precise control of spinning conditions to avoid defect formation.
Interfacial polymerization may be used to prepare composite RO or NF membranes by covering the support layer with a polyamide thin film. Interfacial polymerization of polyamide as the selective layer may be carried out on various polymeric substrates, such as polysulfone (PSF), polyacrylonitrile (PAN), polyvinylidene fluoride (PVDF), and polypropylene.
Specific formula and optimum conditions are needed in order to form a homogeneous covalent bond between the carboxyl group from acyl halides (organic monomer) and the amine group from amine solutions (aqueous monomer). The amide groups are very sensitive to water molecules, as they can weaken the adhesion to the membrane support layer. Moreover, polyamide can be easily attacked by chlorine in water and/or waste water treatment which causes degradation of amide group.
In view of the above, there remains a need for improved nanofiltration membranes and methods to manufacture the nanofiltration membranes that address or at least alleviate one or more of the above-mentioned problems.